The present application relates to a light-transmissive optical sheet typically used as a functional film for liquid crystal display devices, and a backlight device and a liquid crystal display device provided with the same.
Liquid crystal displays (LCD) devices have larger potentials of reduction in power consumption, downsizing and thinning as compared with cathode ray tubes (CRT), and those of a variety of sizes are widely applied at present to small-sized instruments such as mobile phones and digital cameras, up to large-sized liquid crystal television sets.
The liquid crystal display devices are classified into those of transmission type and reflection type. In particular, the transmission type liquid crystal display device has a liquid crystal display panel composed of a liquid crystal layer held between a pair of transparent substrates and a backlight unit as a light source for illumination. The backlight unit is classified into those of direct backlight having a light source disposed directly next to the liquid crystal display panel, and those of edge-light type.
In the backlight unit used for the liquid crystal display devices, beam-condensing optical sheets or films (generally referred to as “sheets”, unless otherwise specifically noted) such as prism sheets or lens sheets aimed at aligning the direction of emission of light from the light source to the front direction are generally used so as to improve front luminance. See Japanese translations of PCT international publication for patent application No. 2001-524225.
In recent years in the field of liquid crystal display devices, there have been advancing trends in improvement in quality of displayed image. Improvement in the luminance is indispensable for improving the image quality. There is, therefore, a demand of further improving the luminance characteristics of the beam-condensing optical sheets represented by the prism sheet.
In the field of liquid crystal TV in recent years, there have been increasing trends in expanding the screen size, and this consequently demands increase in the size of the beam-condensing optical sheets represented by the prism sheet. The optical sheets only as thick as those generally used will, however, fail in obtaining necessary levels of rigidity or stiffness, when the screen size grows up to 50 inches or larger, resulting in deflection and making proper handling difficult. It is also anticipated that lowering in the rigidity of the optical sheets may degrade workability in assembly of backlight devices or liquid crystal display devices, and may make it more likely to cause deflection (waviness) due to heat of the backlight, and this may cause disturbance in the beam-condensing characteristics, and may consequently degrade the image quality.